Capacitive fingerprint sensor

ABSTRACT

On the basis of the physical principle that a capacitance value is inversely proportional to the distance between capacitive electrodes, the spatial structure of the surface of an object can be imaged by measuring a coupling capacitance between the surface of measured object and the electrode arrays on a surface of a sensor; for example, imaging may be performed to uneven spaces between ridge lines and valley lines of fingerprints. The present application provides a C-Q-T type capacitive fingerprint sensor. Firstly, coupling capacitance differences between the fingerprints and the electrodes of the sensor are converted into charge quantity differences, then the charge quantity differences are converted into time differences, and edge signals carrying the time differences are output. Fingerprint sensors are grouped into an array, reading and data combination may be performed to the edge signals, and imaging may be performed to fingerprints.

CROSS REFERENCE OF THE RELATED APPLICATIONS

This application claims priority of the Chinese patent application No.201410004072.3, filed on Jan. 16, 2014 and with the title of “CapacitiveFingerprint Sensor”, which is incorporated herein by reference in itsentirety.

TECHNICAL FIELD

This application relates to a capacitive fingerprint sensor, andparticularly to a capacitive fingerprint sensor that may form an arrayfor imaging fingerprints.

BACKGROUND ARTS

The technology of imaging fingerprints by measuring the differencesbetween the coupling capacitance formed between ridgelines offingerprints and planar sensing electrode array units and that formedbetween valley lines of the fingerprints and the planar sensingelectrode array units is first seen in the patent application U.S. Pat.No. 4,353,056A (Siemens, 1980). Over the past 30 years, sensortechnologies for imaging fingerprints based on the measurement ofcoupling capacitances are continuously developed. Famous enterprisesinvolving this filed include Siemens, AT & T Bell, Philips, Toshiba, ST,NEC, Motorola, Sharp, Intel, Epson and countless venture capitalcompanies.

Most of the capacitive fingerprint sensor technologies are developedbased on macro-capacitive sensor circuit prototypes. However, thetechnical laws of sensors determine that capacitive sensors areacombination of circuits and sensor equations, and the circuit scales andthe implementation processes of circuits determine the value ranges andthe tolerance ranges of various parameters in the sensor equations. Whenthe technical solutions originally formed by macro-scale electroniccomponents are formed by micro-scale electronic components instead, mostof such technical solutions will deteriorate in sensitivity andperformance in noise characteristics.

Further, capacitive fingerprint sensors as array sensors are alsosensitive to the mismatching among the units, which is more difficult tocontrol for micro-scale electronic components with respect tomacro-scale electronic components. Meanwhile, since a measurementcircuit is often large in size, array sensors are generally designed forsingle-channel, and the respective units reuse the measurement circuitin a time division manner. In order to ensure a certain image framerate, prolonging the sampling time, which is an important method forimproving signal gain in the design of array sensors, is greatlylimited, or even becomes unfeasible for a single-channel array sensorwith a relatively large number of points.

Such technical restrictions determine that only a handful of technicalroutes have the potential to move towards commercialization, and suchtechnical routes that do not meet objective technical laws will movetowards extinction. By 2013, there are three types of capacitivefingerprint sensors with certain scale of application: a radio frequencyresponse type, which measures the amplitude of a reflected RF signal,may be represented by a US company Authentec (U.S. Pat. No. 5,828,773A)acquired by Apple Inc. in 2012, and is mainly used iniPhone5S of AppleInc.; a transient response type, which measures a transient couplinglevel, may be represented by a Swedish company Fingerprint Cards(US20080069413A1), and is mainly used in the teller systems of China'sstate-owned banks; and a charge transfer type, which may be representedby a China's Taiwanese company Egistec (U.S. Pat. No. 7,099,497B2), andis mainly used in ideaPad of Lenovo company. The former two types arecalled as active types in the industry, and the third type a passivetype. The common feature of the three types is that a capacitance isconverted into a voltage for measurements. From the view ofclassification of sensors, these three types are classified as “C-V”type sensors.

In recent years, in CMOS photosensors with millions of pixels, to meetthe requirements for growing image point numbers, such photosensorsdevelop from single-channel photosensors to multi-channel ones. Theso-called multi-channel array sensor, in fact, is a plurality ofindependent single-channel array sensors combined spatially. Taking intoaccount the limitations of the actual circuit layout, as the number ofmeasurement circuits changes from one into a plurality, the sizes of themeasurement circuits must be greatly reduced. A “V-T” ADC is a newanalog-digital converter. Compared with a direct type ADC, a “V-T” ADChas the advantage of a greatly reduced circuit scale under the sameresolution requirement, and has the disadvantage of longer sampling andholding time. Multi-channel CMOS sensors tend to adopt “V-T” converters,and balance among the number of channels, the sampling and holding timeand the circuit size so as to determine the best design solution.

The invention patent application filed by Chengdu Microarray ElectronicCo., Ltd. in 2012 with the title of “capacitive distance sensor” and theapplication number of CN201210403271.2 discloses a novel” C-V-T “typecapacitive distance sensor. The method adopted by the above sensor is asbelow: fingerprints are placed on the surface of an object to bemeasured (equivalent to the target electrode); a measuring capacitance(equivalent to the target capacitance) is formed by coupling between thecapacitance measuring plate (equivalent to the sensing electrode) andthe surface of an object to be measured; the distances from the surfaceof the object to be measured (equivalent to the target electrode) to thecapacitance measuring plates (equivalent to the sensing electrode) ofdifferent array units are different, and so do the measuringcapacitances (equivalent to the target capacitance) formed by coupling;a capacitive coupling plate (equivalent to the driving electrode) isdriven by a first programmable level generator (equivalent to the leveldriver) so that the potentials of the capacitance measuring plates(equivalent to the sensing electrode) increase, and the increase degreesof the potentials are different as the measuring capacitance (equivalentto the target capacitance) connected in parallel are different; areference capacitor (equivalent to the integrating capacitor) is firstfully charged, and then discharges to the capacitance measuring plates(equivalent to the sensing electrode) with increased levels; and thedischarging degrees are different as the potentials of the capacitancemeasuring plates (equivalent to the sensing electrode) are different;discharging is repeated so that the potential of the reference capacitor(equivalent to the integrating capacitor) is continuously decreased; asthe decrease rates are different, the time for generating a thresholdlevel by a second level generator (equivalent to the reference level) isdifferent, so that the comparator outputs flip at different timings; thenumber of discharging times corresponding to the time the comparatoroutputs the flip is an output of the capacitive distance sensor.

The measuring function of the sensor is deemed as “S-C-V-T”. That is, adistance from the fingerprints to a sensing electrode is an independentvariable, and the counted value in the time direction is the value ofthe function. The measuring function has the advantages of anti-shiftingand linearity. By taking advantage of improved resolution and reducedthermal noise, the sensor has greatly improved performance. Tape-outverification results show that the technical level of the sensor ishigher than the internationally advanced level in 2012 under the sameprocess conditions, but lower than the internationally advanced level in2013 or the technical level of Touch ID sensors installed in iPhone5S.

Due to active research, development and application of fingerprintsensor technologies by Apple Inc., the consumer electronics market hashuge demand and higher requirements for fingerprint sensor technologies.The present application has improved the technical solution disclosed bythe patent application CN201210403271.2 by establishing and analyzingequations for the sensitivity of sensors. The present application hasintroduced more items and corresponding circuits for increasingsensitivity, while proposing general models for some circuits andsimplifying some circuits.

SUMMARY

One objective of the present application is to provide a capacitivefingerprint sensor with improved sensitivity compared with thecapacitive distance sensor disclosed in the patent applicationCN201210403271.2.

To realize the above objective, the present application provides a“C-Q-T” type capacitive sensor circuit including a “C-Q” converter and a“Q-T” converter. The objective of fingerprint imaging is to performfingerprint recognition, and what is concerned is spatial differenceamong fingerprint ridge lines and fingerprint valley lines. The spatialdifference component is first converted into a fingerprint, and is usedas a difference of a target capacitance Cg formed by coupling betweenthe target electrode Pg and the sensing electrode Ps. The capacitancevalue difference is converted by the “C-Q” converter into a differencein an amount of charge on the sensing electrode Ps. The “Q-T” convertersequentially transfers the charges on the sensing electrode Ps to theintegrating capacitor Ct, so that the integrating capacitor Ct ischarged/discharged, and the charge amount difference is converted into adifference in charging/discharging rates. As the charging/dischargingrates are different, the number of charging/discharging times by whichthe potential Vt of the integrating capacitor Ct changes from an initiallevel to a threshold level Vref3 are different. The comparator comparesthe Vt and the Vref3. The number of charge transfers corresponding tothe time points at which transition edges occur at the output terminalof the comparator is the quantized capacitance value. The transitionedge is output at the output terminal of the comparator according to thecapacitive fingerprint sensor of the present application, read out by acorresponding reading circuit, and converted to the number of chargetransfer times. The “edge time readout circuit” disclosed by the patentapplication filed by Chengdu Microarray Electronic Co., Ltd. in 2012with the title of “capacitive distance sensor” and the applicationnumber of CN201210405080.X may be used for this purpose.

The “C-Q” converter includes: a target electrode Pg (fingerprints), asensing electrode Ps, a driving electrode Pd, a first level driver, asecond level driver, a first initialization switch and a first referencelevel. The capacitor coupling the sensing electrode Ps and the targetelectrode Pg is called a target capacitor Cg. The capacitor coupling thesensing electrode Ps and the driving electrode Pd is called a drivingcapacitor Cd. The capacitor coupling the sensing electrode Ps and abackground circuit (such as a substrate) is called a backgroundcapacitor Cb. As the background capacitor Cb and the driving capacitorCd are located inside the sensor and are connected in parallel, tosimplify the presentation, the background capacitor Cb is regarded as apart of the driving capacitor Cd, and a voltage on the driving capacitorCd is regarded as an average of the voltages of the background capacitorCb and the driving capacitor Cd weighted by their respective capacitancevalues. A first port of the first initialization switch is connected tothe sensing electrode Ps, the first level driver to the target electrodePg, the second level driver to the driving electrode Pd, the firstreference level outputs an initialization level Vref1 and is connectedto a second port of the first initialization switch. The control logictime sequence of the “C-Q” converter is as below:

Step 1-1: the first level driver outputs a level V11 to the targetelectrode Pg; the second level driver outputs a level V21 to the drivingelectrode Pd;

Step 1-2: turn on the first initialization switch, and connect thesensing electrode Ps to the first reference level;

Step 1-3: turn off the first initialization switch;

Step 1-4: the first level driver outputs a level V21 to the targetelectrode Pg; the second level driver outputs a level V22 to the drivingelectrode Pd;

Step 1-5: return to Step 1-1.

In Step 1-2, the potential of the sensing electrode Ps is theinitialization level Vref1.

In Step 1-4, as the sensing electrode Ps is suspended in the air, thepotential changes of the target electrode Pg and the driving electrodePd couple the charges into the sensing electrode Ps. The potential ofthe sensing electrode Ps at the time in Step 1-4 is defined as Vs; then,according to the law of charge conservation, the following equationholds:

(Vref1−V11)*Cg+(Vref1−V21)*Cd=(Vs−V12)*Cg+(Vs−V22)*Cd   (1).

The Equation (1) may be converted into:

(Vs−Vref1)*(Cd+Cg)=(V12−V11)*Cg+(V22−V21)*Cd   (2).

It is defined that ΔV1=V12−V11, ΔV2=V22−V21, and

Vs−Vref1=(ΔV1*Cg+ΔV2*Cd)/(Cd+Cg)   (3).

To facilitate the substitution, the Equation (3) may be converted intoan expression form of Vs:

Vs=(ΔV1*Cg+ΔV2*Cd)/(Cd+Cg)+Vref1   (4).

The “Q-T” converter includes: the integrating capacitor Ct, the secondinitialization switch, the second reference level, the charge transferswitch, the comparator and the third reference level. The integratingcapacitor Ct is connected to a first port of the second initializationswitch, a second port of the charge transfer switch and a first inputterminal of the comparator. The second reference level outputs aninitialization level Vref2, and is connected to a second port of theinitialization switch. A first port of the charge transfer switch isconnected to the sensing electrode Ps of the “C-Q” converter. The thirdreference level outputs a threshold level Vref3, and is connected to asecond input terminal of the comparator. The control logic time sequenceof the “Q-T” converter is as below:

Step 2-1: turn on the second initialization switch, and connect theintegrating capacitor Ct to the second reference level;

Step 2-2: turn off the second initialization switch;

Step 2-3: if the “C-Q” converter is in Step 1-4, lock the “C-Q”converter in Step 1-4;

Step 2-4: turn on the charge transfer switch;

Step 2-5: turn off the charge transfer switch;

Step 2-6: release locking of the “C-Q” converter;

Step 2-7: if the “C-Q” converter leaves Step 1-4, return to Step 2-3.

The first input terminal of the comparator is connected to theintegrating capacitor Ct, the second input terminal of the comparator tothe third reference level, so that the potential Vt of the integratingcapacitor Ct is compared with the threshold level Vref3 output by thethird reference level, and a comparison result is output as an output ofthe “Q-T” converter or an output of the “C-Q-T” converter.

The time information T carried by the output of the sensor is anintegration result of a charge transfer iterative process; T (Cg) doesnot have a strong solution, and a weak solution expression formula hasno analysis value, and can only be analyzed by indirect expressionformulas. The level of the integrating capacitor Ct is defined as Vt. Vtchanges gradually from Vref2 to Vs. The value of T (Cg) is taken suchthat the integration of Vt′ (Cg) on T is the interval length of δ(Vs−Vref2), where δ <1. For a given quantized bit length, T′ (Cg) cannotbe increased by indefinitely increasing T (Cg), so the relative value ofT′ (Cg)/T (Cg) should be increased. Since the highest order term of theweak solution of the mapping from Vt′ (Cg) to T (Cg) is an exponentialdecreasing function, the mapping from Vt′ (Cg) to T (Cg) is a monotonicdecreasing function and the mapping from Vt″ (Cg) to T′ (Cg) is also amonotonic decreasing function. Vt″ (Cg)Vt′ (Cg) may be used to replaceT′ (Cg)/T (Cg) for sensitivity analysis.

Order the integrating capacitor Ct becomes Vt+Vt′ from Vt via chargetransfer for once. Due to the law of conservation of charge, thefollowing equation holds:

(Vt+Vt′)*(Ct+Cd+Cg)=Vt*Ct+Vs*(Cd+Cg)   (5),

which may be converted into:

Vt′=(Vs−Vt)*(Cd+Cg)/(Ct+Cd+Cg)   (6).

The equation (4) is substituted into equation (6) to obtain:

Vt′(Cg)=((ΔV2+Vref1−Vt)*Cd+(ΔV1+Vref1−Vt)*Cg)/(Ct+Cd+Cg)   (7).

By obtaining the derivation of Cg, the equation (7) is simplified into:

Vt″(Cg)=((ΔV1+Vref1−Vt)*Ct+(ΔV1−ΔV2)*Cd)/(Ct+Cd+Cg)̂2   (8).

The influence direction and order over Vt′ (Cg) and Vt″ (Cg) by changesin a single variable can be determined from equations (7) and (8). Theinfluence over Vs−Vref2 should be taken into account at the same time,so that Vs−Vref2 is kept within a suitable range and the value range ofT (Cg) is desirable.

Vs−Vref2=(ΔV1*Cg+ΔV2*Cd)/(Cd+Cg)+Vref1−Vref2   (9).

With reference to the analysis of equations (7), (8) and (9), andconsidering the characteristics of micro-electronic components,technology platform characteristics and various design constraints,circuit design solutions can be optimized. The equation characteristicsrelating to circuit configurations are as follows.

Generally, Ct>>Cd; if the target capacitance is extremely small, Cd>>Cg.ΔV1 is a major item that affects the numerator in equation (8) (based onCt>>Cd), while it is a minor item that affects the numerators inequations (7) and (9). Therefore, by increasing ΔV1, the sensitivity ofthe sensor can be improved significantly. Compared with the technicalsolution of CN201210403271.2, the most important technical improvementis that the first level driver is introduced to couple charges from thetarget electrode Pg to the sensing electrode Ps. Compared with the casewhere ΔV1==0, the equation (8) is approximately increased by(ΔV1+Vref1−Vt)/(Vref1−Vt) times.

Equations (7), (8) and (9) only record ΔV1 and ΔV2, which are notrelated to the absolute values of V11, V12, V21 and V22. Therefore,direct current can be separated by capacitors and only alternatingcurrent (AC) components can be passed between the first level driver andthe target electrode Pg, and between the second level driver and thedriving electrode Pd. In particular, the target electrode Pg may acquirea ΔV1 AC component. The same effect can be achieved by coupling thesensor ground level with an AC level component reverse to ΔV1.

After analyzing equations (7), (8) and (9), it is concluded that theinfluence of ΔV2 is far smaller than that of ΔV1, and even if ΔV2 isconstantly equal to 0, the decrease in sensitivity of the sensor isquite small. Therefore, the present application allows the second leveldriver to be connected to the sensor ground level. This is the essentialdifference between a “C-Q” type sensor and a “C-V” type sensor. If thefirst programmable level generator of the circuit disclosed byCN201210403271.2, which corresponds to the second level driver of thepresent application, is also connected to the sensor ground level, the“C-V” section will always output a ground level.

By establishing and analyzing sensor equations, the inventor hasproposed the circuit implementation solution and the implementationscope of a “C-Q-T” type capacitive fingerprint sensor. The capacitivefingerprint sensor of the present application is configured to form anarray for imaging fingerprints, wherein when a group is formed in asingle-channel form, a plurality of “C-Q” converters reuse one “Q-T”converter in a time division manner, and when a group is formed in amulti-channel form, a plurality of single-channel “C-Q-T” sensor groupswork in parallel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of the capacitive fingerprint sensor of thepresent application;

FIG. 2 is a diagram showing the switch control signals and level controlsignals of the present application;

FIG. 3 is a diagram showing the comparison between the potential curvefamily of the integrating capacitor and the threshold level of thepresent application;

FIG. 4 shows a first embodiment of the first level driver of the presentapplication;

FIG. 5 shows a second embodiment of the first level driver of thepresent application;

FIG. 6 shows a third embodiment of the first level driver of the presentapplication;

FIG. 7 shows a first embodiment of the second level driver of thepresent application;

FIG. 8 shows a second embodiment of the second level driver of thepresent application;

FIG. 9 is a first diagram showing the grouping of the capacitivefingerprint sensor of the present application;

FIG. 10 is a second diagram showing the grouping of the capacitivefingerprint sensor of the present application; and

FIG. 11 is a third diagram showing the grouping of the capacitivefingerprint sensor of the present application.

DETAILED DESCRIPTION

As shown in FIG. 1, the circuit provided by the present applicationincludes: a target electrode 1, a sensing electrode 2, a drivingelectrode 3, a first level driver 4, a second level driver 5, a firstinitialization switch 6, a first reference level 7, an integratingcapacitor 8, a second initialization switch 9, a second reference level10, a charge transfer switch 11, a comparator 12 and a third referencelevel 13, wherein:

the sensing electrode 2 comprises one or more sensing electrodes, and isconnected to a first port of the first initialization switch 6 and afirst port of the charge transfer switch 11;

the target electrode 1 is a surface of a target to be measured, isconnected to the first level driver 4 and is positioned above thesensing electrode 2, wherein a target capacitance 21 is formed betweenthe target electrode 1 and the sensing electrode 2;

the driving electrode 3 includes one or more driving electrodes, isconnected to the second level driver 5 and is positioned below thesensing electrode 2, wherein a dielectric layer is formed between thedriving electrode 3 and the sensing electrode 2, and a drivingcapacitance 23 is formed between the driving electrode 3 and the sensingelectrode 2;

the first level driver 4 is connected to the target electrode 1;

the second level driver 5 is connected to the driving electrode 3;

the first port of the first initialization switch 6 is connected to thesensing electrode 2, and a second port of the first initializationswitch 6 to the first reference level 7;

the first reference level 7 is connected to the second port of the firstinitialization switch 6;

the integrating capacitor 8 includes one capacitor or a plurality ofcapacitors connected in parallel, and is connected to a first port ofthe second initialization switch 9, a second port of the charge transferswitch 11 and a first input terminal of the comparator 12;

the first port of the second initialization switch 9 is connected to theintegrating capacitor 8, and a second port of the second initializationswitch 9 to the second reference level 10;

the second reference level 10 is connected to the second port of thesecond initialization switch 9;

the first port of the charge transfer switch 11 is connected to thesensing electrode 2, and the second port of the charge transfer switch11 to the integrating capacitor 8;

the first input terminal of the comparator 12 is connected to theintegrating capacitor 8, a second input terminal of the comparator 12 tothe third reference level 13, and an output terminal of the comparator12 is an output terminal of the sensor; and

the third reference level 13 is connected to the second input terminalof the comparator 12.

FIG. 2 shows the time sequence between the switch control signal and thelevel control signal:

Step 1: turn on the first initialization switch 6, turn off the chargetransfer switch 11, turn off the second initialization switch 9, set afirst level control signal 413 to low and set a second level controlsignal 523 to low;

Step 2: turn on the second initialization switch 9;

Step 3: turn off the second initialization switch 9;

Step 4: turn off the first initialization switch 6;

Step 5: set the first level control signal 413 to high and set thesecond level control signal 523 to high;

Step 6: turn on the charge transfer switch 11;

Step 7: turn off the charge transfer switch 11;

Step 8: set the first level control signal 413 to low and set the secondlevel control signal 523 to low;

Step 9: turn on the first initialization switch 6;

Step 10: return to Step 4.

FIG. 3 is a diagram showing the comparison between the potential curvefamily of the integrating capacitor 8 and the third reference level 13.The values of the target capacitance 23 are different, so that thepotential change curves of the integrating capacitor 8 fall in differentcurves shown in the curve family, and projections of the intersectingpoints between the potential change curve of the integrating capacitor 8and the third reference level 13 on the time axis are different. Thecomparator 12 compares the two, flips at the intersecting time pointshown in FIG. 3 and outputs a transition edge.

FIG. 4 shows a first embodiment of the first level driver 4 of thepresent application. The first level driver 4 includes an input levelV11 411, an input level V12 412, a first level control signal 413, afirst level selector 414 and a resistor 415. The input level V11 411 isconnected to a first input terminal of the first level selector 414; theinput level V12 412 to a second input terminal of the first levelselector 414; the first level control signal 413 to a control terminalof the first level selector 414; an output terminal of the first levelselector 414 to a first port of the resistor 415; and a second port ofthe resistor 415 to the target electrode 1.

The first level selector 414 outputs a first input terminal level whenthe first level control signal 413 is low, and outputs a second inputterminal level when the first level control signal 413 is high.

FIG. 5 shows a second embodiment of the first level driver 4 of thepresent application. The first level driver 4 includes the input levelV11 411, the input level V12 412, the first level control signal 413,the first level selector 414 and a capacitor 416. The input level V11411 is connected to the first input terminal of the first level selector414; the input level V12 412 to the second input terminal of the firstlevel selector 414; the first level control signal 413 to the controlterminal of the first level selector 414; the output terminal of thefirst level selector 414 to a first electrode of the capacitor 416; anda second electrode of the capacitor 416 to the target electrode 1.

The first level selector 414 outputs the first input terminal level whenthe first level control signal 413 is low, and outputs the second inputterminal level when the first level control signal 413 is high.

FIG. 6 shows a third embodiment of the first level driver 4 of thepresent application. The first level driver 4 includes the first levelcontrol signal 413, a phase inverter 417, a signal converter 418, adriving circuit 419 and a sensor ground level input terminal 420. Thefirst level control signal 413 is connected to an input terminal of thephase inverter 417; an output terminal of the phase inverter 417 to aninput terminal of the signal converter 418; an output terminal of thesignal converter 418 to a control terminal of the driving circuit 419;an output terminal of the driving circuit 419 to the sensor ground levelinput terminal 420; and the target electrode 1 is suspended in the airor grounded.

The signal converter 418 is configured to convert an input signal from asensor ground level domain to a system ground level domain.

The driving circuit 419 is configured to amplify an input terminal leveland provide driving at the output terminal.

FIG. 7 shows a first embodiment of the second level driver 5 of thepresent application. The second level driver 5 includes an input levelV21 521, an input level V22 522, a second level control signal 523 and asecond level selector 524. The input level V21 521 is connected to afirst input terminal of the second level selector 524; the input levelV22 522 to a second input terminal of the second level selector 524; thesecond level control signal 523 to a control terminal of the secondlevel selector 524; and an output terminal of the second level selector524 to the driving electrode 3.

The second level selector 524 outputs a first input terminal level whenthe second level control signal 523 is low, and outputs a second inputterminal level when the second level control signal 523 is high.

FIG. 8 shows a second embodiment of the second level driver 5 of thepresent application. The second level driver 5 includes the input levelV21 521. The input level V21 521 is connected to the driving electrode3.

FIG. 9 is a first diagram showing the grouping of the capacitivefingerprint sensor of the present application. The target electrode 1,the sensing electrode 2, the driving electrode 3, the first level driver4, the second level driver 5, the first initialization switch 6 and thefirst reference level 7 form a first unit circuit 111; and theintegrating capacitor 8, the second initialization switch 9, the secondreference level 10, the charge transfer switch 11, the comparator 12 andthe third reference level 13 form a first reused circuit 112, wherein:the first unit circuit 111 is connected to a first port of a first groupof selectable switches 113, a second port of the first group ofselectable switches 113 to a first bus 114, and the first bus 114 to thefirst reused circuit 112.

The first group of selectable switches 113 is a one-dimensional ortwo-dimensional switch group, and at most one switch is turned on at anytime.

FIG. 10 is a second diagram showing the grouping of the capacitivefingerprint sensor of the present application. The target electrode 1,the sensing electrode 2, the driving electrode 3, the first level driver4, the second level driver 5, the first initialization switch 6, thefirst reference level 7 and the charge transfer switch 11 form a secondunit circuit 121; and the integrating capacitor 8, the secondinitialization switch 9, the second reference level 10, the comparator12 and the third reference level 13 form a second reused circuit 122,wherein: the second unit circuit 121 is connected to a first port of asecond group of selectable switches 123, a second port of the secondgroup of selectable switches 123 to a second bus 124, and the second bus124 to the second reused circuit 122.

The second group of selectable switches 123 is a one-dimensional ortwo-dimensional switch group, and at most one switch is turned on at anytime.

FIG. 11 is a third diagram showing the grouping of the capacitivefingerprint sensor of the present application. The target electrode 1,the sensing electrode 2, the driving electrode 3, the first level driver4, the second level driver 5, the first initialization switch 6, thefirst reference level 7 and the charge transfer switch 11 form a thirdunit circuit 131; the integrating capacitor 8, the second initializationswitch 9 and the second reference level 10 form a primary reused circuit132; and the comparator 12 and the third reference level 13 form asecondary reused circuit 133, wherein: the third unit circuit 131 isconnected to a first port of a primary group of selectable switches 134,a second port of the primary group of selectable switches 134 to a thirdbus 135, the third bus 135 to the primary reused circuit 132, theprimary reused circuit 132 to a first port of a secondary group ofselectable switches 136, a second port of the secondary group ofselectable switches 136 to a fourth bus 137, and the fourth bus 137 tothe secondary reused circuit 133.

The primary group of selectable switches 134 includes a plurality ofone-dimensional switch groups, and at most one switch is turned on foreach switch group at any time.

The secondary group of selectable switches 136 is a one-dimensionalswitch group, and at most one switch is turned on at any time.

The secondary reused circuit 133 may comprise a plurality of comparators12 paired with respective third reference levels 13. The respectivethird reference levels are different from each other.

The present application is not limited to the above embodiments, butincludes all the combinations for realizing the disclosed circuit, whichmay be inferred based on the principle discussed in the presentdescription. All modifications made within the spirit of the presentapplication and the scope of the claims shall fall into the scope of thepresent application.

1-14. (canceled)
 15. A capacitive fingerprint sensor, comprising: atarget electrode, a sensing electrode, a driving electrode, a firstlevel driver, a second level driver, a first initialization switch, afirst reference level and a charge transfer switch, wherein: the sensingelectrode comprises one or more sensing electrodes, and is connected toa first port of the first initialization switch and a first port of thecharge transfer switch; the target electrode is a surface of a target tobe measured, is connected to the first level driver and is positionedabove the sensing electrode, wherein a dielectric layer is formedbetween the target electrode and the sensing electrode, and a targetcapacitance is formed between the target electrode and the sensingelectrode; the driving electrode comprises one or more drivingelectrodes, is connected to the second level driver and is positionedbelow the sensing electrode, wherein a dielectric layer is formedbetween the driving electrode and the sensing electrode, and a drivingcapacitance is formed between the driving electrode and the sensingelectrode; the first level driver is connected to a first level controlsignal and the target electrode; the second level driver is connected toa second level control signal and the driving electrode; the first portof the first initialization switch is connected to the sensingelectrode, and a second port of the first initialization switch to thefirst reference level; the first reference level is connected to thesecond port of the first initialization switch; the first port of thecharge transfer switch is connected to the sensing electrode; the firstlevel driver outputs to a sensor ground inverse AC components of a levelV11 and a level V12.
 16. The capacitive fingerprint sensor of claim 15,wherein the first level driver outputs the level V11 when the firstlevel control signal is low, and outputs the level V12 when the firstlevel control signal is high.
 17. The capacitive fingerprint sensor ofclaim 15, wherein the second level driver outputs a level V21 to thedriving electrode when the second level control signal is low, andoutputs a level V22 to the driving electrode when the second levelcontrol signal is high.
 18. The capacitive fingerprint sensor of claim15, wherein the second level driver outputs the level V21 to the drivingelectrode.
 19. The capacitive fingerprint sensor of claim 15, furthercomprising an integrating capacitor, a second initialization switch anda second reference level, the integrating capacitor comprises onecapacitor or a plurality of capacitors connected in parallel, and isconnected to a first port of the second initialization switch, a secondport of the charge transfer switch; the first port of the secondinitialization switch is connected to the integrating capacitor, and asecond port of the second initialization switch to the second referencelevel; the second reference level is connected to the second port of thesecond initialization switch; and the second port of the charge transferswitch is connected to the integrating capacitor.
 20. The capacitivefingerprint sensor of claim 19, further comprising a comparator and athird reference level, wherein: the first input terminal of thecomparator is connected to the integrating capacitor, a second inputterminal of the comparator to the third reference level, and an outputterminal of the comparator is an output terminal of the sensor; theintegrating capacitor is connected to a first input terminal of acomparator, and the third reference level is connected to the secondinput terminal of the comparator.
 21. The capacitive fingerprint sensorof claim 20, wherein, when an array of sensors is formed, each unitcircuit is formed by the target electrode, the sensing electrode, thedriving electrode, the first level driver, the second level driver, thefirst initialization switch and the first reference level and a reusedcircuit is formed by the integrating capacitor, the secondinitialization switch, the second reference level, the charge transferswitch, the comparator and the third reference level.
 22. The capacitivefingerprint sensor of claim 20, wherein, when an array of sensors isformed, each unit circuit is formed by the target electrode, the sensingelectrode, the driving electrode, the first level driver, the secondlevel driver, the first initialization switch, the first reference leveland the charge transfer switch and a reused circuit is formed by theintegrating capacitor, the second initialization switch, the secondreference level, the comparator and the third reference level.
 23. Thecapacitive fingerprint sensor of claim 22, wherein, when an array ofsensors is formed, each unit circuit is formed by the target electrode,the sensing electrode, the driving electrode, the first level driver,the second level driver, the first initialization switch, the firstreference level and the charge transfer switch, a primary reused circuitis formed by the integrating capacitor, the second initialization switchand the second reference level, and a secondary reused circuit is formedby the comparator and the third reference level.
 24. The capacitivefingerprint sensor of claim 23, wherein the secondary reused circuitcomprises the comparator paired with the third reference level or aplurality of comparators paired with respective third reference levels;and when the secondary reused circuit comprises a plurality of thecomparators paired with the respective third reference levels, therespective third reference levels are different from each other.
 25. Acapacitive fingerprint sensor, comprising: a target electrode, a sensingelectrode, a driving electrode, a first level driver, a second leveldriver, a first initialization switch, a first reference level, anintegrating capacitor, a second initialization switch, a secondreference level and a charge transfer switch, wherein: the sensingelectrode comprises one or more sensing electrodes, and is connected toa first port of the first initialization switch and a first port of thecharge transfer switch; the target electrode is a surface of a target tobe measured, is connected to the first level driver and is positionedabove the sensing electrode, wherein a dielectric layer is formedbetween the target electrode and the sensing electrode, and a targetcapacitance is formed between the target electrode and the sensingelectrode; the driving electrode comprises one or more drivingelectrodes, is connected to the second level driver and is positionedbelow the sensing electrode, wherein a dielectric layer is formedbetween the driving electrode and the sensing electrode, and a drivingcapacitance is formed between the driving electrode and the sensingelectrode; the first level driver is connected to a first level controlsignal and the target electrode; the second level driver is connected toa second level control signal and the driving electrode; the first portof the first initialization switch is connected to the sensingelectrode, and a second port of the first initialization switch to thefirst reference level; the first reference level is connected to thesecond port of the first initialization switch; the integratingcapacitor comprises one capacitor or a plurality of capacitors connectedin parallel, and is connected to a first port of the secondinitialization switch, a second port of the charge transfer switch and afirst input terminal of a comparator; the first port of the secondinitialization switch is connected to the integrating capacitor, and asecond port of the second initialization switch to the second referencelevel; the second reference level is connected to the second port of thesecond initialization switch; and the first port of the charge transferswitch is connected to the sensing electrode, and the second port of thecharge transfer switch to the integrating capacitor; the first leveldriver outputs to a sensor ground inverse AC components of a level V11and a level V12. wherein the first initialization switch, the secondinitialization switch, the charge transfer switch, the first levelcontrol signal and the second level control signal are controlledaccording to the following control sequence: Step 1: turning on thefirst initialization switch, turning off the charge transfer switch,turning off the second initialization switch, setting the first levelcontrol signal to low and setting the second level control signal tolow; Step 2: turning on the second initialization switch; Step 3:turning off the second initialization switch; Step 4: turning off thefirst initialization switch; Step 5: setting the first level controlsignal to high and setting the second level control signal to high; Step6: turning on the charge transfer switch; Step 7: turning off the chargetransfer switch; Step 8: setting the first level control signal to lowand setting the second level control signal to low; Step 9: turning onthe first initialization switch; Step 10: returning to Step
 4. 26. Thecapacitive fingerprint sensor of claim 25, wherein the control sequenceincludes an initialization stage consisting of Steps 1-3 and a chargetransfer stage consisting of Steps 4-10.
 27. The capacitive fingerprintsensor of claim 26, wherein a potential of the integrating capacitorchanges unidirectionally in the charge transfer stage.
 28. Thecapacitive fingerprint sensor of claim 27, further comprising acomparator and a third reference level, the first input terminal of thecomparator is connected to the integrating capacitor, a second inputterminal of the comparator to the third reference level, and an outputterminal of the comparator is an output terminal of the sensor; and thethird reference level is connected to the second input terminal of thecomparator; wherein the potential of the integrating capacitor changesunidirectionally; and when the potential of the integrating capacitorchanges from a level higher than the third reference level to a levellower than the third reference level or changes from a level lower thanthe third reference level to a level higher than the third referencelevel, an output of the comparator is inverted and a sensor output isgenerated.